The invention relates to a display device comprising an image display panel having a first substrate which is provided with electrodes at the area of pixels, and an illumination system comprising an optical waveguide of an optically transparent material having an exit face facing the image display panel and a plurality of end faces, at least one of said end faces, being an entrance face for light, while light can be coupled into said and face of the optical waveguide.
The image display panel may comprise an electro-optical medium (between two substrates) such as a liquid crystal material or an electrochromic material. It may also be based on electrostatic forces (deformable mirrors).
Such reflective display devices are used in, for example, portable apparatus such as laptop computers, mobile telephones, personal organizers, etc. With a view to saving energy, it is desirable that the light source can be switched off in the case of sufficient ambient light.
The invention also relates to an illumination system for use in such a display device.
A display device of the type mentioned above is described in xe2x80x9cCompact Front Lighting for Reflective Displaysxe2x80x9d, SD) 96 Applications Digest, pp 4346. This document shows an optical waveguide having a groove structure at the area of a first main face remote from the image display panel. The groove structure is necessary to deflect light rays into the direction of the image display panel. A problem in this case is that Fresnel reflections occur on various surfaces. In the relevant example, light is reflected on the side of the image display panel but also on faces of the underlying image display panel Qolarizers, retardation filters). Together with the groove structure, the light thus reflected gives rise to Moirxc3xa9 effects (artefacts). Such effects also occur in other optical waveguides with regularly spread parallel reflection patterns. These may be optical waveguides having a groove structure at the area of a second main face facing the image display panel. The optical waveguide may also have integrated reflection patterns, for example, when polarizing means are integrated in the optical wavegaide, in which an unpolarized beam from the light source is split up by polarization into two mutually perpendicularly polarized beam components. Such a polarization separation is obtained, for example, by causing the unpolarized beam to be incident on an interface between an area of isotropic material having a refractive index np and an area of anisotropic material having refractive indices no and ne, in which one of the two indices no or ne is equal or substantially equal to np. When an unpolarized beam is incident on such an interface, the beam component which does not experience any refractive index difference at the transition between isotropic and anisotropic material is passed in an undeflected form, whereas the other beam component is deflected or reflected.
It is an object of the present invention to provide a solution to this problem. To this end, a display device (and an optical waveguide) according to the invention is characterized in that the optical waveguide has reflection patterns in the light path, which reflection patterns extend at least partly at an angle to the end face.
The phrase stating that the grooves xe2x80x9cextend at least partly at an angle to the end facexe2x80x9d does not necessarily exclude the presence of parallel grooves.
A first embodiment of a display device (and an optical waveguide) according to the invention is characterized in that the reflection patterns are curved with respect to the end face. Viewed in a direction parallel to the end face, the reflection patterns may have an iterative pattern, for example, a pattern of corrugations.
A second embodiment of a display device (and an optical waveguide) according to the invention is characterized in that the optical waveguide has a plurality of short reflection patterns which are mutually oriented in an arbitrary and different way.
Such reflection patterns do not only considerably reduce Moirxc3xa9 artefacts caused by the above-mentioned reflections but also Moirxc3xa9 patterns which are caused by interference phenomena due to reflection of light on a regular pattern in the display panel and transmission of light through an optical waveguide having parallel grooves. It is to be noted that EP-A-940 706 provides a solution to the last-mentioned problem (namely, by suitably choosing the mutual spacing between the grooves).
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.